Project Summary The 2018 Physical Activity Guidelines Scientific Report, which summarized the most up-to-date evidence, suggested that, for youth the evidence for the effects of physical activity on obesity prevention is strong but the evidence for the effects of sedentary behavior remains limited. The report recognizes several important knowledge gaps, one of which is regarding the independent and interactive effects of physical activity and sedentary behavior on health outcomes. The abrupt difference in the physical activity guidelines across the transitional stage from youth to adulthood (e.g., daily one hour for youth vs. weekly 2.5 hours for adults) is also recognized as problematic. To address these gaps, we propose to conduct a pooled analysis to quantify the independent and interactive effects of physical activity and sedentary behavior on adiposity in early adulthood. The main hypotheses are (1) a healthier physical activity pattern during childhood and early adulthood is associated with lower adiposity in early adulthood, independent of sedentary behavior, and (2) a healthier sedentary behavior pattern is associated with lower adiposity in early adulthood only among those with a less healthy physical activity pattern. The first aim of this study is to examine the diverse trajectory patterns of moderate- and vigorous-intensity physical activity and sedentary behavior from age 11 to 24 years, using a group-based dual trajectory analysis approach. The second aim is to compare adiposity at age 22-24 years by moderate- and vigorous-intensity physical activity and sedentary behavior trajectory patterns. Two sub-aims are: to examine whether healthier physical activity and/or sedentary behavior patterns are associated with a lower risk of obesity at age 22-24 years among children with early childhood obesity; and to examine a dose- response relationship of moderate- and vigorous-intensity physical activity and sedentary behavior with fat mass change in the youth to adult transition period (from age 15-17 to 22-24 years). To achieve these aims, we will use pooled data from three birth cohort studies: U.K. Avon Longitudinal Study of Parents and Children, Brazil 1993 Pelotas Study, and U.S. Iowa Bone Development Study. The pooled data will enable us to have a diverse sample and sufficient power for the subgroup analysis. This low-cost self-contained R03 project is expected to produce new knowledge to update the Physical Activity Guidelines by addressing these knowledge gaps.